1. Technical Field
The present invention relates to a control device of a secondary battery, and, in particular, to a correction technique of a map (or a table) for calculating a state-of-charge (SOC) of the secondary battery.
2. Related Art
A secondary battery such as a nickel-metal hydride battery or a lithium ion battery is used as a power supply for supplying electric power to a driving motor or the like of an electric vehicle or a hybrid electric vehicle. In order to efficiently use the secondary battery, a current state of the secondary battery must be accurately evaluated.
JP 2005-37230 A discloses a technique in which an estimated charging/discharging current and a measured charging/discharging current are compared, and degradation of the secondary battery is detected based on the comparison result.
JP 2007-113953 A discloses a technique in which a voltage drop percentage during a non-usage period is calculated based on a voltage at the start of the non-usage period of the vehicle and a voltage at the end of the non-usage period, and the voltage drop percentage is compared with a reference value, to judge degradation of the secondary battery.
JP 2008-14702 A discloses a technique in which a charging/discharging current is integrated to calculate a remaining capacity based on a current integration, a remaining capacity is calculated based on an estimated value of an open voltage of the secondary battery, and a combined remaining capacity is calculated by combining the remaining capacities with certain assigned weights. Based on an amount of change of the remaining capacity based on the current integration and an amount of change of the combined remaining capacity, a current capacity change percentage is calculated, and, when the current capacity change percentage becomes less than or equal to a predetermined value, degradation of the secondary battery is determined.
In general, when a no-load voltage which is a voltage intercept when the current is zero in the secondary battery is OCV, a certain relationship exists between the no-load voltage OCV and the SOC of the secondary battery. Therefore, by storing, in a storage device in advance, a two-dimensional map which defines the relationship between the no-load voltage OCV and the SOC, calculating the no-load voltage based on a current and a voltage of the secondary battery, and referring to the two-dimensional map, a current SOC of the secondary battery can be calculated. In this specification, the no-load voltage is defined as OCV.
However, the relationship between the no-load voltage OCV and the SOC changes due to aging deterioration of the secondary battery, and, thus, a difference may arise between the SOC calculated using the two-dimensional map stored in the storage device and the actual SOC, and, if the secondary battery is controlled based on the calculated SOC, the secondary battery cannot be efficiently controlled, and, moreover, the degradation of the secondary battery may be accelerated.